1. Field of the Invention
The present invention pertains notably to a combined electric and optic cable for applications relating notably to high definition television (HDTV). The invention also relates to an application of the combined cable to the link between a camera head and a control unit.
2. Description of the Prior Art
A standard way of connecting a camera head to a control unit is to use an electric triaxial cable, namely a shielded coaxial cable This cable has the following characteristics:
it is designed to transmit all the video signals as well as the service signals, such as the audio, interphone, remote control and servocontrol signals, by frequency division multiplexing on the internal coaxial structure: each signal to be transmitted modulates a different carrier frequency. These carrier frequencies are multiplexed and the resulting complex signal is conveyed on the internal coaxial structure of the triaxial cable; PA1 it transmits the supply of the camera head; PA1 it transmits particular service signals, known as "back-up operation" signals, used when the equipment is not in normal operation; PA1 it enables the grounding of the camera head by an external conductive braid which also serves as a shielding for the cable. The range of this type of cable is great (typically 1 to 1.5 km). However, a standard triaxial cable such a this attenuates the high frequency signals far too excessively to enable the transmission of high definition television signals; the typical degree of attenuation is 3.9 dB for 100 m of cable at 60 MHz. Now, for high definition television, the passbands of the video signals to be transmitted are far bigger than for the presently used television signals. The passbands are typically equal to 30 MHz or even 60 MHz instead of 5 MHz as at present. Furthermore, if the signals are digitalized and transmitted digitally, the bit rate of the binary information in series is far greater: it is greater than 1 Gbit.
There are also cables with optic fibers only. These cables are used for cable networks. However, they have no electrical conductor for the supply, the back-up signals and the grounding. They are therefore not appropriate for high definition television applications.
There are also multiple-conductor applications that have been used in the past for certain cameras. A link achieved by a cable such as this is bidirectional and each signal to be transmitted is conveyed by a different conductor. However, their technology is dated. They have a small range (300 to 600 m); they are brittle and unreliable, and take up a substantial amount of space.
Finally, there are combined coaxial/optic fiber cables used for sonars. In these cables, the insulator of the coaxial cable is grooved to insert optic fibers. These combined cables may include up to ten optic fibers. However, their structure is not suited to the applications envisaged because it is not rotationally symmetrical and is very similar to the structure of multiple-conductor cables.